In our commonly assigned prior U.S. Pat. Nos. 3,978,435 and 3,984,789 we have disclosed and claimed an equalizer for optimizing the recovery of data pulses from an incoming message stream. Reference may also be made in this connection to our article entitled "A state-variable approach to the equalization of multilevel digital signals", CSELT Rapporti Tecnici No. 3, April 1974. Such a digital equalizer utilizes the principle of recursive filtration first enunciated by R. E. Kalman in an article entitled "A New Approach to Linear Filtering and Prediction Problems", Journal of Basic Engineering/Transactions of the ASME, March 1960. The algorithm described in our prior patents involves the generation of an updating signal and the weighting of the incoming data pulses by coefficients which are determined by the transmission characteristics of the message channel and which may be initially established in an acquisition phase preceding an operating phase.
Systems are also known for deriving the timing information for the demodulation of an incoming carrier from the data pulses with which this carrier is modulated by phase-shift keying, with utilization of in-phase and quadrature signal components as well as differentiation. Reference may be made in this connection to U.S. Pat. No. 3,634,773 in the name of Hisashi Kobayashi and to an article by that inventor entitled "Simultaneous Adaptive Estimation and Decision Algorithm for Carrier-Modulated Data-Transmission Systems", IEEE Transactions on Communication Vol. COM-19, No. 3, pages 268 - 280, June 1971. The Kobayashi system employs adjustable equalizers of the so-called "transverse-filter" type rather than those with recursive filtration described in our prior patents.
With such a transverse filter there is only a single data pulse available in the output of the equalizer during any clock cycle. Since the establishment of synchronism without the transmission of special synchronization signals requires the correlation of data pulses sampled in a multiplicity of consecutive clock cycles, such a system introduces considerable delays which are particularly disadvantageous in the presence of several feedback loops interacting with one another during an acquisition phase, i.e. prior to steady-state operation. Aside from lengthening the acquisition phase, such interaction may also result in instability of the receiver.